Method of casting synthetic organic compounds



G. B. TAYLOR MET flOD OF CASTING SYNTHETIC ORGANIC COMPOUNDS Filed March9,- 1939 |NVENTQR v Y ATTORNE Patented Decals, 1942 estates METHOD .OFCASTING SYNTHETIC ORGANIC Coll/[POUNDS Guy B. Taylor, Wilmington, Del.,assignor to E. I.

du Pont de Nemours & Company, Wilmington, Del., a corporation ofDelaware Application March 9, 1939, Serial No. 260,692

6 Claims.

This invention relates to a method of casting synthetic organiccompounds and, more particularly, to a method of casting moltensynthetic linear superpolymers.

The term superpolymer is used herein as in Carothers U. S. Patent2,071,250 to refer to synthetic polymers capable of being formed intouseful fibers. As disclosed in the aforementioned patent, such polymersinclude polyesters, polyamides, polyethers, and polyanhydrides. Thesepolymers are microcrystalline in character and can beconverted intofibers showing by characteristic X-ray patterns orientation along thefiber axis.

The term "superpolyamides is used herein to refer to polyamides capableof being formed into useful fibers. Such polyamides and theirpreparation are disclosed in Carothers U. S. Patents 2,071,253 and2,130,948, and are designated generically in the art as nylon. In thesepolyamides the amide groups form an integral part of the main chain ofatoms inthe'polymer.

Generally, thesesuperpolymers are not plastics but, rather, havedefinite melting points near their decomposition temperatures, and aresubject to oxidation when exposed to air at temperatures approachingtheir decomposition temperatures; further, there is in many cases alarge contraction in volume of material as the molten superpolymerssolidify. It will be obvious that these superpolymers cannot be cast bythe usual methods heretofore employed in the plastics art. Moreover, themolding-powder technique of the plastics art is not practical with thesesuperpolymers not only for the reasons above but also because they arenot easily reduced to pulverulent form.

In order to cast these superpolymers acceptably they must be cast at orabove their melting point. An additional requirement in the case oi manysuperpolymers is the rigid exclusion oi? air to prevent oxidation. Manyof these superpolymers, particularly the superpolyamides, present afurther problem in that they tend to decompose slightly at temperatureshigh enough to melt them and, as a result, bubbles may be formed therebypreventing a sound casting. Not all of the superpolyamides are subjectto this bubble formation when in molten condition but in many instancesthe tendency to bubble formation is sufliciently great to present aserious problem.

An object of the present invention is to provide a satisfactory methodof casting these superpolymers. A further object is to provide a methodwherein superpolyamides subject to bubble formation in the molten statemay be satisfactorily cast. A still further object is to provide amethod of casting flawless rods of superpolymers for use in meltspinning according to the process disclosed in U. S. Patent No.2,253,089, in the name of Roland R. Nydegger, entitled "Spinningapparatus and method, assigned to the assignee of the presentapplication. A further object of the invention is to provide a method ofcasting superpolymers in turnery shapes adapted for machining, and thelike. Other objects will be apparent from the description of theinvention given hereinafter.

The above objects are accomplished according to the present invention bycooling, in the presence of an inert atmosphere, a molten superpolymerdisposed in a mold having a mold cavity and a superposed reservoircavity directly connecting with the mold cavity, the reservoir cavitybeing greater in horizontal cross sectional area than the upper portionof the mold cavity, under such conditions that a core of moltensuperpolymer connecting with the mold cavity is maintained in thereservoir cavity until the superpolymer in the mold cavity issolidified, the amount of superpolymer disposed in the mold being atleast suificient to fill the mold cavity and a part of the reservoircavity after it has been allowed to solidify.

More particularly, the present invention contemplatescastingsuperpolymers which are subject to bubble-formation when molten, byincreasing the pressure upon the molten material in the mold until allbubbles formed are suppressed, that is, collapsed or dissolved in the.

melt, and thereafter cooling the molten superpolymer as above.

The invention generally is carried out by introducing superpolymer,usually in the form of flakes or lumps, into the mold which is providedwith means for introducing an inert gas, heating the mold until thesuperpolymer melts, and, then, subjecting the molten superpolymer topressure to suppress any bubbles formed in it and cooling the mold inthe manner described.

Alternatively, the monomeric ingredients may be introduced into themold, the polymerization reaction carried out in such a way to end withthe superpolymer in moltencondition, and then the subsequent procedurecarried out in the same manner as if superpolymer in the solid state hadbeen introduced into the mold and melted.

Referring to the drawing accompanying the present application:

Fig. 1 is a vertical section of a mold suitable for casting rodsaccording to the present invention; and

Figs, 2-5, inclusive, are vertical sections of castings made in the moldillustrated in Fig. 1, under varying conditions.

Referring to Fig. l, themold there shown comprises a mold cavity ladapted for casting rods and a reservoir cavity 2 superposed on the moldcavity i and communicating directly therewith. It will be noted that thehorizontal cross sectional area the reservoir cavity 2 exceeds that ofthe mold cavity I. In this'instance, the horizontal cross sectional area01' the mold cavity l is substantially identical throughout its heightbut, in the event that it varied, it would be sufficient for the crosssectional area or the reservoir cavity simply to exceed that of theupper portion of the mold cavity.

The upper portion of the mold is externally threaded and the cap 3 isadapted to be screwed thereon. This cap 3 is provided with the pipeconnection 4 which may be connected with a vacuum line or with anysource of gaseous pressure whereby air in the mold can be displaced byan inert gas and/or the mold contents placed under pressure.

The method of cooling the molten superpolymer is an important feature ofthe present in vention and, in order to obtain a sound casting, must becarried out in such a manner that a core of molten superpolymerconnecting with the mold cavity is maintained in the reservoir cavityuntil the superpolymer in the mold cavity is solidified. To accomplishthis, it is obvious that care must be taken in the manner in which themold is cooled, the manner in which the mold has been heatedinfluencing, to some extent, the manner in which the mold may be cooledto obtain the desired result.

It the mold is filled with superpolymer which on being melted andresolidified fills the mold cavity and the reservoir cavity to the levelindicated by the arrow in Fig. 1, the method of heating the mold to meltthe superpolymer will greatly aflect the results when the cooling iscarried out in certain ways. For example, the molten superpolymer willbe heated to the same temperature throughout whether the mold 'is heatedby (1) immersion in a liquid bath to a level Just above that of themolten superpolymer, or (2) immersing the whole mold including the coverin a bath. However, in case (1) on cooling the mold in air, a pipeappears in the casting as shown in Fig. 2, thus giving an unsatisfactorycasing whereas, in case (2) on'spontaneous cooling in the air, no pipeappears in the casting proper but frequently a void will occur in thecone of superpolymer which solidified in the reservoir cavity, as shownin Fig. 3. As the object is to secure only the rod as a sound casting,the result in this instance is satisfactory,

In case (2) above, it cooling is hastened by lowering the mold intowater to a levelabout half the height of the mold cavity l, a sharpertemperature gradient is established so that the top is forced tosolidify last and the casting is sound throughout, see Fig. 4; thisresult may also be obtained in case (1) ii the cooling by means of wateris carefully done to avoid premature solidincation of the center of themass in the reservoir cavity.

It will be understood that, in the above illustrations, the reason forthe difference in results between case (1) and (2) is largelyattributable to the fact that the mold, particularly that portion of itforming the walls of the reservoir cav-- ity, absorbs more heat in case(2) and, hence, tends to keep the superpolymer in the reservoir cavitymolten for a period sufllcient to permit solidification of thesuperpolymer in the mold cavity.

The reason that the reservoir cavity 2 is an essential feature of a moldsuitable for carrying out the present invention is illustrated in Fig. 5which shows a section of a rod cast in a straight tube mold not having asuperposed reservoir cavity. In this instance the solidification of thesuperpolymer adjacent the walls in the upper portion of the mold leavesno molten material to fill the pipe formed due to contraction of thematerial as it solidifies and this is true no matter what method ofcooling is used. By having the reservoir cavity considerably greater inhorizontal cross sectional area than the mold cavity and, also, ofappreciable height, a molten core of superpolymer in the reservoircavity is quite readily maintained and permits considerable variation ofthe conditions under which the mold may be cooled to obtain a soundcasting. On-

the other hand, 11 cooling'oi the mold is carried out with great care tokeep a substantial temperature diflerentiation between the material inthe mold cavity and in the reservoir cavity, then it .is not necessarythat the horizontal cross sectional area of the reservoir cavity besubstantially greater than that of the upper portion of the mold cavity,in fact, they could be substantially the same.

Obviously, the cover of the mold, when heated, assists in maintainingthe molten core of superpolymer in the reservoir cavity by restrictingloss of heat through radiation from the upper surface of thesuperpolymer in the reservoir cavity.

The following examples are given to illustrate specific embodiments ofthe present invention:

Example I.-A mold adapted for casting rods and having the relativedimensions as the mold shown in Fig. 1 was used. The diameter of thereservoir cavity was 3" and the diameter of the mold cavity was 1%".This mold was charged with flake polymeric hexamethylene adipamide in anamount sufilclent to give a melt level at the point indicated by thearrow in Fig. 1. The

air in the mold was displaced by nitrogen, the mold sealed by closing avalve in the gas line connecting with the mold, and the mold was thenimmersed entirely in a fused salt bath maintained at 280 C. until all ofthe polymer was melted.

The melt within the mold at this stage contains numerous small bubblesdue to slight decomposition of the polymer. To suppress these bubblesthe nitrogen pressure within the mold was raised to 300 pounds persquare inch. The mold was then removed from the bath and allowed to coolin the air, the pressure being relieved alter the polymer solidified.When the moldhas cooled to room temperature, it is opened and themeniscus contact of the polymer with the walls of the mold is cut with aknife. The molded piece then drops freely out upon tuming the moldupside down. A sound casting was thus obtained.

Example II.The same mold as used in Example I was charged with 230 gramsof. hexamethylene diammonium adipate and grams of water and set in anelectric furnace with heating elements extending only as far as theasoasee cap for the mold. Heating was started and shortly after steamappeared, showing displacement of the air, a valve cutting of? the pipeconnection to the atmosphere was closed. Thereafter, heating wascontinued until 250 pounds per square inch gage pressurewas reached.Steam was bled out at this pressure until the mold contents reached atemperature of 270 0., then the pressure was slowly released at constanttemperature down to atmospheric pressure. The temperature was raised to280 C. and held for one hour, an auxiliary .heater being applied to heatthe cap cover to the same temperature during this period. Nitrogenpressure oi. 200 pounds per square inch was applied-and the mold thenlifted from the furnace and allowed to cool. in air. A casting wasobtained such as illustrated in Fig. 3, that is, there was a void in thecone of material which solidified in the reservoir cavity but this in noway ailected the soundness of the rod casting.

Example III.The procedure of Example 11 was followed except that thecooling was done by immersing the lower half of the mold in water.

In this case the casting was sound throughout and as indicated in Fig.4.

The above examples illustrate the invention as applied to superpolymersthat are not plastics, have a fairly sharp melting point, a largecontraction on solidification, and exhibitno tendency for the casting tostick to the mold. Other specific superpolymers of this type includehexamethylene adipamide-terephthal amide interpolymer andhexamethylene-betamethyl adipamide polymer.

There are some superpolymers which do not so clearly exhibit theproperties mentioned above and, as a consequence, tend to stick in themold. The present invention is equally applicable to such superpolymersexcept that a glass or thin metal liner,is inserted in the mold andbroken or machined of! the finished casting. The following exampleillustrates this:

Example IV.---An interpolymer was prepared from an-equi-molar mixture ofthe salts hexamethylene diammonium adipate and decamethylene diammoniumsebacate by the procedure of Example 11 in a mold provided with analuminum liner. A flawless casting was obtained and the thin metal linerremoved therefrom by machining in a lathe.

Amide-ester interpolymers frequently require a linerin the mold toprevent sticking; hexamethylene diarnmonium isophthalate is anothermaterial that tends to stick in the mold and generally necessitates theuse of a liner.

The above examples are merely illustrative 'both as' to the specificsuperpolymers disclosed and the specific method employed. The inventionis applicable to superpolymers broadly and is especially advantageous inthe casting of superpolymers that have relatively definite meltingpoints not far removed from their decom-.

position temperatures, that have a tendency toward bubble-formation whenmolten, and have an unusually large shrinkage factor in solidify- Thesuperpolyamides in general are freing. quently desired in the form ofcastings and the present invention is particularly useful for theproduction of castings of this class of superpolymers.

v The hereindisclosed procedure is not restricted to castings ofcylinders. Any shape may be molded providing the mold is constructed sothat there is a reservoir adapted to feed down molten shape issuperpolymer to avoid-the formation oi a Where a shape having other thanstraight sides is to be cast, it is necessary, of course, to strip themold ad the casting.

The present invention is useful in the preparaticn of cast articles fromsuperpolymers, especlally where turnery shapes are desired and flaw"less rods for use in the procedure disclosed in the aforementionedNydegger Patent 23o. 2,253,089.

An advantage of the present invention is that it provides a practicalmethod of producing cash ings under gas pressure from a melt thatrequires handling in a narrow temperature range set by the melting pointand decomposition temperature. A further advantage of the-invention isthat it permits the production of flawless castings adapted for turneryp poses and other uses where flaws cannot be tolerated. A still furtheradvantage of the present invention is that it does not involvecomplicated or costly apparatus nor exceedingly fine adjustment ofconditions such as would be beyond the scope of an ordinary laborer.

As many apparently widely difierent embodiments of this invention may bemade without departing from the spirit and scope thereof, it is to beunderstood that the invention is not limited to the specific embodimentsthereof except as defined in the appended claims.

I claim:

1. A process of casting voidless shaped articles of syntheticsuperpolymers which comprises disposing in an amount of moltensuperpolymer suficient when subsequently solidified to fill a cavity ofthe desired shape at the bottom of said mold and to at least partiallyfill a directly connecting superposed reservoir cavity having arelatively larger horizontal cross-sectional area than the upper portionof said shaped cavity, and in the presence of said inert gas cooling themolten superpohmier in the cavity of the desired shape more rapidly thanthe superpolymer in the reservoir cavity subjecting the lower portion ofsaid mold to a cooling medium whereby a core of molten super polymerconnecting with the cavity of desired maintained in the reservoir cavityuntil the superpolymer of the desired shape is solidified.

2. A process of casting voidless shaped articles of syntheticsuperpolyamides which comprises disposing in a mold in the presence ofan inert gas. an amount of molten superpolyamide sufi= cient whensubsequently solidified to fill a cavity of the desired shape at thebottom of said mold and to at least partially fill 2. directlyconnecting superposed reservoir cavity having a relatively largerhorizontal cross-sectional area than-the upper portion of said shapedcavity, increasing the pressure of inert gas in the mold until bubblesformed by decomposition of the molten superpolyamide are suppressed, inthe presence of said inert gas under pressure cooling the moltensuperpolyamide in the cavity of the desired shape more rapidly than thesuperpolyamide in the reservoir cavity by subjecting the lower portionof said mold to a cooling medium whereby a core of molten superpolyamideconnecting with the cav ity of desired shape is maintained in theresenvoir cavity until the superpolyamide of the desired shape 'issolidified, then releasing the pressure of inert gas in said mold.

3. A process of casting voidless shaped articles of syntheticsuperpolymers which comprises disa mold in the presence of an inert gasposing in a mold an amount of solid superpolymer sufficient whensubsequently melted and resolidified to fill a cavity of the desiredshape at the bottom of said mold and to at least partially fill adirectly connecting superposed reservoir cavity having a relativelylarger horizontal cross-sectional area than the upper portion of saidshaped cavity, displacing the air in said mold by an inert gas, thenheating substantially the entire mold until all of said superpolymer ismelted, and in the presence of said inert gas cooling the moltensuperpolymer in the cavity of the desired shape more rapidly than thesuperpolymer in the reservoir cavity by subjecting said mold to acooling medium whereby a core of molten superpolymer connecting with thecavity of desired shape is maintained in the reservoir cavity until thesuperpolymer of the desired shape is solidified.

4. A process of casting voidless shaped articles of syntheticsuperpolymers which comprises disposing in a mold an amount of solidsuperpolymer suflicient when subsequently melted and resolidified tofill a cavity of the desired shape at the bottom of said mold and to atleast partially fill a directly connecting superposed reservoir cavityhaving a relatively larger horizontal crosssectional area than the upperportion of said shaped cavity, displacing the air in said mold by aninert gas, then heating said mold until all of said superpolymer ismelted, and in the presence of said inert gas cooling the moltensuperpolymer in the cavity of the desired shape more rapidly than thesuperpolymer in the reservoir cavity by subjecting the lower portion ofsaid mold to a cooling medium whereby a core of molten superpolymerconnecting with the cavity of desired shape is maintained in thereservoir cavity untilthe superpolymer of the desired shape issolidified.

5. A process of casting voidless shaped articles of syntheticsuperpolyamides which comprises disposing in a mold an amount of solidsuperpolyamide sufiicientwhen subsequently melted and resolidified tofill a cavity of the desired shape at the bottom of said mold and to atleast partially fill a directly connecting superposed reservoir cavityhaving a relatively larger horizontal cross-sectional area than theupper portion of said shaped cavity, displacing the air in said mold byan inert gas, then heating substantially the entire mold until all ofsaid superpolyamide is melted, increasing the pressure of inert gas inthe mold until bubbles formed by decomposition of the moltensuperpolyamide are suppressed, in the presence of said inert gas underpressur cooling the molten superpolyamide in the cavity of the desiredshape more rapidly than the superpolyamide in the reservoir cavity bysubjecting said mold to a cooling medium whereby a core of moltensuperpolyamide connecting with the cavity of desired shape is maintainedin the reservoir cavity until the superpolyamide of the desired shape issolidified, then releasing the pressure of inert gas in said mold. I

6. Aprocess or casting voidless shaped articles of syntheticsuperpolyamides which comprises disposing in a mold an amount of solidsuperpolyamide sufllcient when subsequently melted and resolidifled tofill a cavity of the desired shape at th bottom of said mold and to atleast partially fill a directly connecting superposed reservoir cavityhaving a relatively larger horizontal crosssectional area than the upperportion of said shaped cavity, displacing the air in said mold by aninert gas, then heating said mold until all of said superpolyamide ismelted, increasing the pressure of inert gas in the mold until bubblesformed by decomposition of the molten superpolyamide are suppressed, inthe presence of said inert gas under pressure cooling the moltensuperpolyamide in the cavity of the desired shape more rapidly than thesuperpolyamide in the reservoir cavity by subjecting th lower portion ofsaid mold to a cooling medium whereby a core of molten superpolyamideconnecting with the cavity of desired shape is maintained in thereservoir cavity until the superpolyamide of the desired shape issolidified, then releasing the pressure of inert gas in said mold.

GUY B. TAYLOR.

